This invention relates generally to epoxy-amine systems. More particularly, it relates to radiation-initiated epoxy-amine systems.
The three dimensional epoxy-amine network produced by the curing of amine-epoxy resin formulations is well known to exhibit desirable mechanical and thermal properties. As a result, amine-curable epoxy resin systems have been widely used as coatings, adhesives, sealants and matrices for fiber-reinforce composites. For each application, the epoxy-amine resin formulation must possess a particular degree of reactivity. In many cases, the reaction rate must be increased, and cure accelerators are added.
Normally, the epoxy-amine compositions are marketed as a two-part system with the first part containing the epoxy resin and the second part containing the amine curative and cure accelerator. It is highly desirable to market a one-part system in which the epoxy resin, amine curative, cure accelerator and other components are blended together under controlled conditions or a two-part system with a prolonged pot life and accelerated cure. Such a system could eliminate costly field mixing by the consumer who in many instances does not have the proper equipment to institute a thorough blend of the components and/or a proper appreciation of the need to make and apply a homogeneous mixture of the components for optimum results.
Considerable efforts have been made to improve one-part epoxy-amine resin systems by adding various additives thereto. It has been found that various Lewis acids, Lewis bases, and numerous salts and complexes are added into the epoxy-amine systems as accelerators. Such accelerators, while improving the cure speed, have been found to adversely affect the working life of the system as well as complicate the work with significant volumes of materials.
Various mono- and di-hydroxy substituted aromatic compounds (i.e. phenols, resorcinols and their derivatives) are also effective in increasing the cure speed of glycidyl epoxy-amine mixtures.
Sterically hindered amine curatives, such as substituted toluenediamines, are used to produce epoxy-amine formulations with prolonged pot life (i.e., the length of time during which an adhesive, after being mixed with catalyst and/or curing agent, remains suitable for use). However, conventional formulations with such xe2x80x9cslowxe2x80x9d amines require curing at a temperature of over 150xc2x0 C. for several hours. Therefore, the use of a catalyst is necessary. One of the most common catalysts used in such systems are derivatives of phenol, such as salicylic acid, resorcinol, and nonylphenol. The catalytic activity of these compounds results from the presence of the phenolic hydroxyl groups. Unfortunately, an addition of such catalysts causes the immediate acceleration of the chemical interaction between epoxides and amines, resulting in epoxy-amine formulations with extremely limited shelf life (i.e., the length of time an unopened package of adhesive can be expected to remain in usable condition under specific conditions of temperature and humidity), and very short pot life.
U.S. Pat. No. 4,593,056, issued to Qureshi et al. on Jun. 3, 1986, discloses a curable thermosetting resin composition including an epoxy resin, an aromatic diamine hardener, and an aromatic trihydroxy compound as a cure accelerator. However, the trihydroxy compound causes immediate acceleration of the chemical interaction between epoxides and amines, resulting in extremely limited shelf life and very short pot life of such composition.
U.S. Pat. No. 4,510,290 and 4,636,575, issued to Kirchmayr et al. on Apr. 9, 1985 and Jan. 13, 1987, respectively, disclose a one-component acid-curable composition containing an acid-curable resin and a latent curing catalyst which is activated by the UV radiation. The resin is an amino resin or a phenolic resin, or a mixture of an amino resin or a phenolic resin with an alkyd, polyester or acrylic resin.
Kirchmayr""s acid-curable composition further includes a xe2x80x9cblocked UV absorberxe2x80x9d, such as resorcinol monobenzoate or certain salicylic acid esters, which while absorbing UV energy, undergo a Photo-Fries rearrangement. However,
Kirchmayr""s latent curing catalysts are un-suitable for epoxy-amine systems.
U.S. Pat. No. 4,636,431, issued to DeBergalis on Jan. 13, 1987, discloses a coating composition including a polymer containing an epoxy group provided by glycidyl methacrylate and resorcinol monobenzoate. The resorcinol monobenzoate can be bonded to the polymer through hydroxyl group on the resorcinol monobenzoate. The resulting polymer-bound resorcinol monobenzoate, when subjected to ultraviolet radiation, absorbs part of the radiation and undergoes a Photo-Fries rearrangement to produce polymer-bound dihydroxy benzophenone. However, DeBergalis only uses Photo-Fries rearrangement for scavenging UV radiation in cured polymers.
There is a need, therefore, for a one-component epoxy-amine composition having a prolonged shelf and/or pot life.
Accordingly, it is a primary object of the present invention to provide an epoxy-amine composition having a prolonged shelf and/or pot life.
It is another object of the present invention to provide an epoxy-amine composition floating for a sufficiently long time after the initiation and curing, thus allowing processing of large bodies with complicated profiles, including the filling of narrow channels.
It is a further object of the present invention to provide an epoxy-amine composition useful for the preparation of premixes, prepregs, laminates and composites, etc.
The present invention is directed to epoxy-amine compositions having prolonged shelf life and/or pot life through the use of xe2x80x9chiddenxe2x80x9d or latent catalysts or curing agents.
An epoxy-amine composition of the present invention includes an epoxy resin, an amine curative, and a catalytic system.
The catalytic system does not cause the instant acceleration of the reaction between epoxides and amines. Rather, in the presence of radiation, such as visible light, ultraviolet (UV) light or an electron beam, the latent catalyst or latent curing agent undergoes photo-Fries rearrangement to generate an active catalyst or active curing agent that accelerates/initiates the cure process of the epoxy-amine composition.
The presence of photosensitizers, such as anthracene, pyrene, and benzophenone, promotes more effective active catalyst generation.
The epoxy resin means compounds with terminal epoxy groups, preferably glycidyl ethers of bis-phenol A and bis-phenol F or novolacs. The amine curative preferably contains a substituted toluenediamine, such as dimethylthiotoluenediamine and diethyltoluenediamine. The catalyst preferably consists of esters of phenols and esters of derivatives of phenols, such as acetylsalicylic acid, resorcinol monoacetate, resorcinol monobenzoate, diacetate of bis-phenol A, phenyl acetate, phenyl benzoate, naphtyl acetates, and resorcinol diacetate.
The epoxy-amine composition further can include a proton-accepting compound containing oxygen atoms for inhibiting the interaction among the catalyst, epoxides and amines. The proton-accepting compound preferably includes 2,4,6-trimethylbenzoyldiphenylphosphine oxides, alpha-hydroxyketone, benzophenone derivatives, and 1,3-dioxolane.
The epoxy-amine composition can also include a Lewis acid or a compound that generates a Lewis acid upon the exposure to radiation.
The presence of acid scavengers, preferably cycloaliphatic epoxy resins, can significantly prolong the pot life of the epoxy-amine system.
Prior to its exposure to radiation, the epoxy-amine compositions of the present invention have a prolonged pot life. After exposure, the speed of curing of the epoxy-amine composition is significantly increased.